1. Field of the Invention.
The subject invention relates generally to computerized tomography, and more particularly to an improved system for providing tomographic images of subject wherein a computerized tomographic system is controlled in accordance with object density information and digitized mapping information determined from a preliminary image of the subject.
2. Prior Art.
In this field it is desirable to obtain radiographic images of controlled depth and slice thickness for accurate diagnostic purposes. In performing tomographic imaging of objects, it is desirable to avoid repeated irradiation of the object. However, obtaining the correct positioning and the correct exposure levels for tomographic studies frequently required making repeated exposures of the object in order to iteratively correct the positioning and the exposure level. This method required exposing the object to a radiation dose which is relatively high.
A conventional X-ray computerized-tomographic apparatus could display both a preliminary image such as a scanogram as well as a tomographic image. Thus, before acquisition of the tomographic images of the target portion of the object under examination, the scanogram, which covered a wide region including the diseased portion, was produced. The scanogram was based upon an X-ray transmission detection signal. By observing the scanogram, positions for imaginary slices of the subject for which the tomographic images were to be obtained were determined. The tomographic images were obtained by reconstructing other detection signals based upon the determined slice positions.
In systems such as this, a control device controlled motors for the translatory displacement of the carrier in the horizontal plane so that the pivoting axis of the carrier followed a curve which at least substantially corresponded to the shape of the subject. The drive motor for the carrier pivots the carrier so that the connecting line between the focal spot of the radiation source and a slit diaphragm of the film holder through which the radiation is incident upon the film extends approximately perpendicularly with respect to the curve. The film is transported within the film holder at a velocity which is proportional to and higher than the translation velocity of the carrier. Alternately, the subject may be held stationary and only the portions of the tomographic system translated.
However, inaccuracies and, hence unsharpness and distortion may still occur. Schrieber, U.S. Pat. No. 4,418,419 issued on Nov. 29, 1983, teaches a solution to this problem wherein a control device included an element which was applied to the subject. The control device also included an arithmetic unit for calculating control signals for the drive motor. A tomographic system was then controlled in accordance with the control signals from the control device. However, this control device added considerable expense and still did not achieve correct exposures at all points of the translation of the system.
It is also known to provide computerized tomographic systems wherein a patient was subjected to a scanographic scanning operation prior to the computer tomograph scanning operation, to specify or determine a tomographic scanning portion for a displayed X-ray image. For the preliminary scanning operation, an X-ray tube and an X-ray detector array of the X-ray CT apparatus, were arranged in a fixed position and the patient translated along a longitudinal axis between the X-ray tube and the X-ray detector. However, this method often required undesirably high levels of irradiation of the subject and movement of the subject may result in less precise placement of the subject with respect to the tomographic system.
In the prior art the scanogram was displayed on a CRT apparatus and lines through the image were arranged. After seeing the scanogram, an operator specified tomographic scanning portions and pressed the keys of a keyboard to indicate where in the object the scan slice should be taken. Automatic positioning of the source of X-ray irradiation of the object for CT scanning on the specified portions of the scanographic image in a predetermined order could then proceed. Such a system is taught in U.S. Pat. No. 4,649,555, issued to Matsubayashi on Mar. 10, 1989. However, the system of Matsubayashi still did not provide correct exposures for the different possible studies of selected locations of the object, thereby often requiring extra irradiation of the object.
The most common solution to this problem was to have an operator make a series of measurements of the subject in accordance with the scanographic image. A series of complex, trigonometric, geometric, and arithmetic operations based on the measurements were performed by the operator to determined the remaining information required for correct exposure once the tomographic system was properly positioned around the subject. Performance of these operations required a highly trained operator and were still subject to error. When errors were made, the exposures were incorrect and additional irradiation of the subject was required at a corrected level of irradiation.
Such earlier methods required that certain anatomical structures be identified from the radiographic image, previously made of the subject. The physical dimensions were determined from this image using manual drawing aids, such as rulers, protractor, and straight-edge in order to calculate the required relative subject to X-ray position. This required a working knowledge of trigonometry as well as special training for the operator of the X-ray equipment to determine the patient to equipment relationship. Usually, the first attempt resulted in missed tomographs or at best images that were incorrectly exposed thereby subjecting the patient to undesirable additional radiation and the inconvenience of repeated lengthy and costly procedures.